In real life, more complex multicellular microbes also play major roles, for example in the chemistry of the environment and as pathogenic (disease-producing) agents. Fungi (singular, fungus) are particularly significant. They are eukaryotes and can be classified as follows: microscopic, and macroscopic (multicellular). Two basic features are common to multicellular microfungi: (i) under the microscope, cells are observed to occur in the form of threadlike filaments that often have branches, and (ii) the filamentous cell masses produce special reproductive structures that shed spores in great abundance. These features are illustrated by the scanning electron micrograph of the mould Penicillium shown in this chapter. Fungi are commonly observed growing in colorful patches on tree trunks or barren materials such as bare rocks and house roofs. A number of fungi are pathogenic for animals and plants. Fungal infections of humans and other animals are usually called mycoses. Important examples include thrush, diaper rash, vaginitis, and the lung diseases coccidioidomycosis (caused by Coccidioides immitis) and histoplasmosis (Histoplasma capsulatum). The ability of many fungi to grow on materials of low nutrient content explains why they frequently accumulate in buildings, causing nasal and eye irritation or respiratory distress to many people. As viewed under the microscope, Phytophthora infestans shows the typical features of eukaryotic microscopic fungi. Owing to an obscure technicality, it is now frequently referred to as "fungus-like". Leaf-cutter ants in tropical forests apparently invented agriculture long before humans did.

Scanning electron micrograph of Penicillium roqueforti, the blue mould used in the manufacture of roquefort cheese. Note the resemblance of the fruiting body (bearing chains of spores, or conidia) to a paint brush. The Latin for “paint brush” is penicillus.

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Figure 21

Scanning electron micrograph of Penicillium roqueforti, the blue mould used in the manufacture of roquefort cheese. Note the resemblance of the fruiting body (bearing chains of spores, or conidia) to a paint brush. The Latin for “paint brush” is penicillus.